1. Technical Field of the Invention
The present invention relates in general to the field of adaptive quantization of digitized images, and in particular, to JPEG adaptive-quantization signaling.
2. Description of Related Art
The Joint Photographic Experts Group image-compression standard (JPEG) is used in many digital imaging systems. JPEG is the most widely-used method for still image compression. The most-commonly-used JPEG mode is referred to as the “baseline sequential mode.” A less-commonly used mode of JPEG is the “progressive mode.” A baseline-sequential JPEG file is stored as a single scan of an image. JPEG progressive mode divides the image into a series of scans. A first scan shows the image at an equivalent of a very low quality setting. Following scans gradually improve quality of the displayed image. Each scan adds to the data already provided so that the total storage requirement is roughly the same as for a baseline JPEG image of the same quality as the final scan. An advantage of progressive JPEG is that if an image is being viewed on-the-fly as it is transmitted, one can see an approximation of the whole image very quickly, with gradual improvement of quality as one waits longer. A disadvantage is that each scan takes about the same amount of computation to display as a whole baseline JPEG file would.
Various image compression-decompression techniques are used that compress a digitized image according to the JPEG baseline sequential mode. A drawback of the various known compression-decompression techniques used in conjunction with JPEG images is that at high compression ratios, data block boundaries of the image become visible in regions that should appear smooth, and, near edges, ringing appears in the image. It is understood that better data compression results can be achieved for high compression ratios by utilizing adaptive quantization techniques. “Adaptive quantization” refers to adaptively varying a quantization matrix (Q matrix) from data block to data block using an encoder.
While adaptive quantization helps to solve some of the problems discussed above, adaptive quantization cannot be used unless a mechanism for signaling the adaptation to the decoder is devised. The decoder must vary the Q matrix at each data block in exactly the same manner as the encoder in order to obtain the benefits of adaptive quantization.
A number of adaptive-quantization signaling schemes already exist. U.S. Pat. No. 5,157,488 to Pennebaker (Pennebaker) describes the use of an extra “color” component for signaling. Because JPEG can accommodate any number of components (red, green, blue, and possibly others), extra components can be sub-sampled by an integer factor relative to original components. A decoder that can interpret Pennebaker's signaling scheme can be programmed to recognize how the extra component signals a varying Q matrix. However, when separate systems are used for encoding and decoding an image, the decoding system can have a standard encoder rather than one that is “aware” of Pennebaker's signaling scheme. In such a case, the extra component would be decoded incorrectly, limiting the applicability of Pennebaker's approach.
U.S. Pat. No. 5,822,458 to Silverstein, et. al (Silverstein) discloses use of the parity of a sum of quantized DCT coefficients in each data block for signaling purposes. After the DCT coefficients are divided by the corresponding elements of the Q matrix, they are rounded to an integer value. Values having fractional portions near to 0.5 can be rounded either up or down without incurring significant additional error. For example, 11.499 can be rounded to either 11 or 12, the error being approximately the same in either case. Silverstein discloses taking the coefficient closest to the quantization midpoint and rounding the midpoint up or down so that the parity of the data block is even or odd. The parity therefore signals information that could be used for adaptive quantization.
Silverstein's signaling scheme requires a search of the 64 coefficients of each data block in order to determine the coefficient that is closest to the midpoint, which can result in slower encoding and increased computational load. In addition, if a coefficient with a fractional portion close to 0.5 cannot be found, rounding by amounts up to 1 might be required. In such cases, the rounding errors could result in less-than-desirable results.
What is needed is an adaptive-quantization signaling scheme that does not compromise image quality and retains compatibility with available decoding schemes.